This invention relates to piezoelectric transducers and may be extended to pyroelectric transducers as well, the active element of which is constituted by a polymer in sheet form. When subjected to a suitable treatment, said active element exhibits piezoelectric and pyroelectric properties which are similar to those possessed by certain classes of crystals.
One of the first synthetic polymers to show clear evidence of piezoelectric and pyroelectric properties was polyvinylidene fluoride. In this case the treatment includes: (a) unidirectional drawing of a flat film of said polymer, (b) metallizing the faces of the drawn film and (c) subjecting the metallized faces to an electric field by connecting its metallized faces to an electric polarizing generator. The disadvantage of a flat film lies in the fact that its use is limited to transducers having developable shapes, that is shapes which are capable of being opened and flattened out upon a plane without stretching any element. Furthermore, the thinness of the films employed entails the need for stretching latter by means of a prestressing device.
This disadvantage can be overcome by means of a thermoforming technique which makes it possible to obtain a non-developable self-supporting shape which consequently does not have any joint. The thermoforming operation is carried out at a temperature which results in stretching of the molecular chains since this stretching process is intended to produce a change of phase which makes the material polar. It is then an easy matter to induce electrical anisotropy by polarization. The electrodes are clearly formed after the polymer film has been given its final shape.
The technique which includes inducing the polar phase by substantial drawing of the polymer makes it necessary to take precautions in order to prevent shrinkage of the drawn film or to prevent the shape obtained by thermoforming from shrivelling-up and thus losing its self-supporting properties. During operation, a polymer-film transducer must be capable of withstanding a temperature rise while retaining its shape, its dimensions and its conversion efficiency.
In addition to the lack of dimensional stability resulting from the disturbing effect produced on the mechanical equilibrium by drawing performed above the melting temperature of the polymer, it should also be mentioned that the shapes usually obtained from polyvinylidene fluoride have relatively low mechanical compliance.
In order to overcome these drawbacks, the invention provides a method of manufacture which essentially includes electrically polarizing a shaped product so as to retain its original elastic properties. This does not prevent development of piezoelectric and pryroelectric properties since the electrical anisotropy induced by the polarization is the only factor involved in the transducing action which takes place. By reason of the fact that only the desired anisotropy is electrical and that it is produced by an electric field having a direction perpendicular to the faces of the polymer film, the transducing action is related to the effect of certain crystal systems which have symmetry of revolution with respect to the normal to the faces of the manufactured product.